PROJECT SUMMARY/ABSTRACT The Special Supplemental Nutrition Program for Women, Infants and Children (WIC) is the third largest federal nutrition assistance program in the United States. Established in 1972, it serves low- income pregnant and postpartum women and children ages 0-5 years who are at nutritional risk. WIC costs nearly $6 b, and served about 8 m pregnant and post-partum women and young children in 2016. Despite WIC's wide reach and long history, there is a dearth of evidence supporting the impact of WIC on the health and well-being of its participants and on societal costs. A few studies have examined the impact of prenatal participation in WIC on pregnancy outcomes, and even fewer studies have examined the impact of WIC on the nutritional status, and growth and development of young children. One of the greatest barriers to strengthening the evidence base for WIC's impact is the lack of availability of a scientifically appropriate comparison group, especially among infants and children. Such a comparison group would allow for the design and implementation of a rigorous scientific study (a randomized controlled trial). The proposed study will take advantage of an unusual opportunity to merge a unique database of Los Angeles (LA) County WIC participants with a comparable database of Kaiser Permanente (KP) patients to form such a comparison group. KP is one the largest health- care providers in California, and about 25% of KP patients are on Medi-Cal (California's Medicaid program). By merging these two databases, and developing an innovative approach for identifying WIC-eligible Kaiser patients who are not enrolled in WIC, we will attempt to create a scientifically rigorous comparison group for future evaluations of the impact of the WIC program on young children. To assess the validity of this comparison group (and the protocol developed for forming it), we will survey a randomly selected sample of KP patients to determine if the protocol we develop is adequately sensitive and specific. We will focus on children ages 2-5 years because there is a dearth of evaluation research on the impact of WIC on child growth and development. The findings of this study will have implications for future funding decisions affecting programs that support the growth, development and nutritional status of children from disadvantaged families.